This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2000-95080 filed on Mar. 30, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a motor yoke and a method of manufacturing the same.
2. Description of Related Art
Conventionally, a yoke of a motor is manufactured by plastically deforming a metal plate (iron plate). The manufacturing processes are generally drawing processes (press workings) which have a plurality of steps such as a first drawing step, subsequent drawing steps and a final forming step. In the first drawing step, the plate is drawn to a nearly dome shaped body. In the subsequent drawing steps, the dome shaped body is further drawn in a manner that a diameter thereof is gradually reduced. After the yoke is formed in rough shape according to the steps mentioned above, its minor portions are further deformed to manufacture the yoke in final shape with the final forming step.
In more details, as shown in FIG. 10, while a portion 89 of a metal plate 81 around an entire outer circumference of a cavity of a die 84 is held down between a wrinkle preventing plate 83 and the die 84, a punch 85 urges a sheet of the metal plate 81 so as to be pushed into the cavity of the die 84 under constant pressure.
When the metal plate 81 is pushed to extend axially in the cavity at a drawing step, compressive stresses are induced uniformly in a circumferential direction of the metal plate 81 formed in cup shape and tensile stresses are induced uniformly in an axial direction thereof. Accordingly, as shown in FIG. 11A, a thickness T of a yoke, that is, a thickness of a cup shaped product 82 made of the metal plate 81 by drawing, is circumferentially nearly uniform.
Further, a partly flat yoke 88, whose cross section is formed in a generally oval shape and includes parallel flat portions 86 and curved portions 87, is manufactured in such a manner that the cup shaped product 82, which is made of the metal plate 81 at the first drawing step and whose radial thickness is circumferentially uniform, is further drawn by an oval punch through the subsequent drawing steps and, then, finally shaped at the final forming step. As a diameter of the cup shaped product 82 is reduced through the drawing steps, material of the product 82 flows toward the curved portions 87. Accordingly, in the partly flat yoke 88, each thickness T1 of the curved portions 87 becomes thicker and each thickness T2 of the parallel flat portions 86 becomes thinner, as shown in FIG. 11B.
Since permanent magnets 90 are fixed to inner surfaces of the curved portions 87 of the partly flat conventional yoke and each of the curved portions 87 does not play an important role as a magnetic path of the motor, the curved portion 87 having the thicker thickness T2 results in material loss. To the contrary, each of the parallel flat portions 86 plays an important role as the magnetic path. Therefore, the parallel flat portions 86 having the thinner thickness T2 thereof cause to induce higher magnetic resistance so that a motor output (torque) is limited.
To solve the drawback mentioned above, conventionally, thickness of the parallel flat portions 86 have been controlled by employing the metal plate 81 whose original thickness is relatively thick under consideration of the subsequent thickness reduction in the drawing processes or by employing a complicated compression drawing method. However, the drawback that the curved portions 87 are still thicker than that of the parallel flat portions 86 has not been solved. As a result, manufacturing cost is higher due to the material loss and a compact and slim type motor is not realized.
An object of the invention is to provide a method of manufacturing a motor yoke having circumferential walls whose each thickness is relatively thick and to which magnets are to be attached and having circumferential walls whose each thickness is relatively thin and which play important roles as magnetic paths. Accordingly, material of the metal plate is saved and a compact, lightweight and high torque motor may be realized.
It is another aspect of the invention to provide a motor yoke manufactured by the method mentioned above.
To achieve the above objects, in a method of manufacturing a cup shaped motor yoke having circumferential walls at given angular regions to which magnets are to be attached and circumferential walls at remaining angular regions which play roles as main magnetic paths, by drawing a sheet of metal plate, the metal plate is put between a die having a cavity and a holding plate having a through-hole which is axially aligned with the cavity so that the metal plate is held down therebetween at positions around outer circumferences of the cavity and the through-hole corresponding to the given angular regions and the metal plate is placed with a clearance having a predetermined distance from one of the die and the holding plate at positions around the outer circumference of the cavity corresponding to the remaining angular regions. Then, the punch presses down the metal plate through the through-hole into the cavity so that material of the metal plate extends axially into the cavity, while the material of the metal plate flows to move in the cavity circumferentially from each side of the given angular regions to sides of the remaining angular regions. As a result, a first stage yoke, whose each thickness of the circumferential walls at the given angular regions is thinner than that at the remaining angular regions, is completed without having substantial wrinkles thereon.
Preferably, the given angular regions are located symmetrically with respect to an axial center of the cavity.
It is preferable that the metal plate is held down between the die and projections formed on a surface of the holding plate around the through-hole and each height of the projections corresponds to the predetermined distance of the clearance.
Further, for the purpose of preventing generation of wrinkles, it is preferable that the predetermined distance of the clearance falls within a range of 8 to 15 percent of the thickness of the metal plate.
Furthermore, preferably, the first stage yoke is, then, placed and held down between another die having a cavity and another holding plate having a through-hole at entire positions around the cavity and the trough-hole thereof. Diameter of the cavity of the another die is smaller than that of the die and a periphery end of the cavity of the another die on a side of facing the another holding plate has round corners. Each curvature of the round corners at positions corresponding to the given angular regions is larger than that at positions corresponding to the remaining angular regions. Then, another punch, whose diameter is smaller than that of the punch, presses down the first stage yoke through the through-hole of the another holding plate so that the wall thickness of the first stage yoke at the given angular regions become further thinner and the wall thickness thereof at the remaining angular regions become further thicker. As a result, a second stage yoke, whose diameter and depth are smaller and deeper than those of the first stage yoke, respectively, is completed.
Preferably, the curvatures of the round corners are continuously and gradually reduced from each middle point of the given angular regions toward middle points of the remaining angular regions oppositely adjacent thereto.
The cup shaped yoke may be a partly flat yoke whose circumferential wall is composed of two parallel flat walls playing roles as the main magnetic paths and two curved walls to which a pair of the permanent magnet are attached. In this case, it is preferable to manufacture the partly flat yoke with the punches and the cavities of the dies, whose cross sections are formed in oval shape.
Further, the cup shaped yoke may be a nearly cylindrical yoke whose outer shape of the circumferential wall is oval in cross section and whose inner shape of the circumferential wall is circular in cross section. The magnets are attached to inner circumferential walls on opposite end sides thereof in a major axis of oval.